Organic semiconductor structure, process for producing the same, and organic semiconductor device

ABSTRACT

There are provided an organic semiconductor structure comprising an organic semiconductor layer, which is large in size and homogeneous and has high charge transfer characteristics, a process for producing the same, and an organic semiconductor device. The organic semiconductor structure has, in at least a part thereof, an organic semiconductor layer comprising an aligned liquid crystalline organic semiconductor material. The liquid crystalline organic semiconductor material comprises an organic compound having a core comprising L 6 π electron rings, M 8 π electron rings, N 10 π electron rings, O 12 π electron rings, P 14 π electron rings, Q 16 π electron rings, R 18 π electron rings, S 20 π electron rings, T 22 π electron rings, U 24 π electron rings, and V 26 π electron rings, wherein L, M, N, O, P, Q, R, S, T, U, and V are each an integer of 0 (zero) to 6 and L+M+N+O+P+Q+R+S+T+U+V=1 to 6. The liquid crystalline organic semiconductor material exhibits at least one liquid crystal state at a temperature below the heat decomposition temperature thereof.

TECHNICAL FIELD

The present invention relates to an organic semiconductor structurecomprising an organic semiconductor layer formed using a liquidcrystalline organic semiconductor material, a process for producing thesame, and an organic semiconductor device.

BACKGROUND ART

Thin-film transistors (also known as “organic TFT”) utilizing an organicsemiconductor in an active layer (hereinafter referred to as “organicsemiconductor layer”) may be mentioned as typical elements forconstituting organic semiconductor devices.

In the thin-film transistors, the organic semiconductor layer is formedby forming a film of a molecular crystal typified by pentacene in vacuo.Regarding the formation of the organic semiconductor layer by the filmformation in vacuo, there is a report that the optimization of the filmformation conditions can realize the formation of an organicsemiconductor layer having a high level of charge mobility exceeding 1cm²/V·s (Y. -Y. Lin, D. J. Gundlach, S. Nelson, and T. N. Jackson,“Stacked Pentacene Layer Organic Thin-Film Transistors with ImprovedCharacteristics,” IEEE Electron Device Lett. 18, 606 (1997)). Theorganic semiconductor layer formed by the film formation in vacuo,however, is generally in a polycrystal form composed of aggregates offine crystals. Therefore, many grain boundaries are likely to exist,and, in addition, defects are likely to occur. The grain boundaries anddefects inhibit the transfer of charges. For this reason, in theformation of the organic semiconductor layer by film formation in vacuo,the organic semiconductor layer as the element for constituting theorganic semiconductor device could not have been substantiallycontinuously produced in a satisfactory wide area with homogeneousproperties and without difficulties.

On the other hand, a discotic liquid crystal is known as a materialhaving a high level of charge mobility (D. Adam, F. Closss, T. Frey, D.Funhoff, D. Haarer, H. Ringsdorf, P. Schunaher, and K. Siemensmyer,Phys. Rev. Lett., 70, 457 (1993)). In this discotic liquid crystal,however, charge transfer is carried out based on a one-dimensionalcharge transfer mechanism along the columnar molecular alignment.Therefore, close control of the molecular alignment is required, andthis disadvantageously makes it difficult to utilize the discotic liquidcrystal on a commercial scale. Any example of success in thin-filmtransistors using the discotic liquid crystal as a material forconstituting the organic semiconductor layer has not been reported yet.

A high level of charge mobility in a liquid crystal state of rodlikeliquid crystalline materials such as phenylbenzothiazole derivatives hasalready been reported (M. Funahashi and J. Hanna, Jpn. J. Appl. Phys.,35, L703–L705 (1996)). Up to now, however, there is no report on anyexample of success of thin-film transistors utilizing the rodlike liquidcrystalline material in the organic semiconductor layer. The rodlikeliquid crystalline material exhibits a few types of liquid crystalstates, and the charge mobility is likely to increase with enhancing theregularity of the structure of the liquid crystalline material. Thetransition of the liquid crystalline material to a crystal state havinga higher level of regularity of structure, however, results in loweredor no charge mobility. In this case, of course, no properties requiredof the thin-film transistor are developed.

When a molecular dispersion polymeric material is used as an organicsemiconductor material, an organic semiconductor layer, which hasuniform charge transfer characteristics over a large area can be formedby coating this organic semiconductor material. The organicsemiconductor layer thus formed, however, has a low charge mobility of10⁻⁵ to 10⁻⁶ cm²/V·s, and, disadvantageously, the charge mobilitydepends upon temperatures and electric fields.

The present invention has been made with a view to solving the aboveproblems of the prior art, and an object of the present invention is toprovide an organic semiconductor structure comprising an organicsemiconductor layer having a relatively large area and uniform and highlevel of charge transfer characteristics, which have hitherto beenregarded as unattainable, a process for producing the same, and anorganic semiconductor device.

DISCLOSURE OF THE INVENTION

The above object can be attained by an organic semiconductor structurehaving, in at least a part thereof, an organic semiconductor layercomprising an aligned liquid crystalline organic semiconductor material,said liquid crystalline organic semiconductor material comprising anorganic compound having a core comprising L 6 π electron rings, M 8 πelectron rings, N 10 π electron rings, O 12 π electron rings, P 14 πelectron rings, Q 16 π electron rings, R 18 π electron rings, S 20 πelectron rings, T 22 π electron rings, U 24 π electron rings, and V 26 πelectron rings, wherein L, M, N, O, P, Q, R, S, T, U, and V are each aninteger of 0 (zero) to 6 and L+M+N+O+P+Q+R+S+T+U+V=1 to 6, said liquidcrystalline organic semiconductor material exhibiting at least oneliquid crystal state at a temperature below the heat decompositiontemperature thereof. According to another aspect of the presentinvention, there is provided an organic semiconductor structure having,in at least a part thereof, an organic semiconductor layer comprising analigned liquid crystalline organic semiconductor material, said liquidcrystalline organic semiconductor material comprising an organiccompound having a core comprising L 6 π electron rings, M 8 π electronrings, N 10 π electron rings, O 12 π electron rings, P 14 π electronrings, Q 16 π electron rings, R 18 π electron rings, S 20 π electronrings, T 22 π electron rings, U 24 π electron rings, and V 26 π electronrings, wherein L, M, N, O, P, Q, R, S, T, U, and V are each an integerof 0 (zero) to 6 and L+M+N+O+P+Q+R+S+T+U+V=1 to 6, said liquidcrystalline organic semiconductor material exhibiting at least a smecticliquid crystal phase state at a temperature below the heat decompositiontemperature thereof. According to still another aspect of the presentinvention, there is provided an organic semiconductor structure having,in at least a part thereof, an organic semiconductor layer comprising analigned liquid crystalline organic semiconductor material, said liquidcrystalline organic semiconductor material comprising an organiccompound having a core comprising L 6 π electron rings, M 8 π electronrings, N 10 π electron rings, O 12 π electron rings, P 14 π electronrings, Q 16 π electron rings, R 18 π electron rings, S 20 π electronrings, T 22 π electron rings, U 24 π electron rings, and V 26 π electronrings, wherein L, M, N, O, P, Q, R, S, T, U, and V are each an integerof 0 (zero) to 6 and L+M+N+O+P+Q+R+S+T+U+V=1 to 6, said liquidcrystalline organic semiconductor material having, at its both ends, aterminal group capable of developing liquid crystallinity.

In an embodiment of the present invention, preferably, at least a partof said liquid crystalline organic semiconductor material in the organicsemiconductor layer has been aligned and crystallized by holding theliquid crystalline organic semiconductor material at a temperaturesuitable for the conversion of the liquid crystalline organicsemiconductor material to a liquid crystal state and then cooling theliquid crystalline organic semiconductor material.

Further, preferably, the organic semiconductor layer is stacked incontact with a liquid crystal aligning layer, and the provision of theorganic semiconductor layer in contact with the liquid crystal aligninglayer permits the liquid crystalline organic semiconductor material tobe aligned in a specific orientation or direction. In particular,preferably, the organic semiconductor layer is stacked and aligned on aliquid crystal aligning layer formed of a polyimide material, or isstacked and aligned on a liquid crystal aligning layer formed of a curedresin having fine concaves and convexes on its surface, or is formed ona substrate formed of a cured resin having fine concaves and convexes onits surface.

According to a further aspect of the present invention, there isprovided an organic semiconductor structure comprising an organicsemiconductor layer and a liquid crystal aligning layer, said organicsemiconductor layer comprising a liquid crystalline organicsemiconductor material, which exhibits at least one liquid crystal stateat a predetermined temperature below the heat decomposition temperature,and being provided in contact with the liquid crystal aligning layer, atleast a part of the liquid crystalline organic semiconductor materialhaving been aligned and crystallized.

According to the organic semiconductor structure of the presentinvention, the organic semiconductor layer is formed of a liquidcrystalline organic semiconductor material having, at its end (eitherboth ends or one end), a terminal structure (also known as “terminalgroup”) capable of developing liquid crystallinity. Therefore, themolecular alignment is spontaneously realized by the self-organizationof the liquid crystalline organic semiconductor material. The molecularalignment is similar to that of crystals. As a result, excellent chargetransfer characteristic as in molecular crystals can be developed.Further, when a liquid crystalline organic semiconductor material havinga smectic liquid crystal phase of a high order is used, an organicsemiconductor layer having a very high level of crystallinity can beformed. Furthermore, the organic semiconductor material in a liquidcrystal state is fluid at such a temperature that can maintain theliquid crystal state. Therefore, the organic semiconductor material canbe coated in the liquid crystal state, and the coating can be thenbrought to the above crystal state. As a result, a large-area organicsemiconductor layer having uniform charge transfer characteristics canbe formed. In addition, the organic semiconductor layer formed of theliquid crystalline organic semiconductor material has a high level ofcrystallinity by virtue of the molecular orientation order and has avery small intermolecular distance. Therefore, excellent charge transfercharacteristics derived from hopping conduction can be provided.Further, when means for aligning the liquid crystalline organicsemiconductor material is properly selected, liquid crystallinemolecules can be aligned in a specific direction. Therefore,functionalities and electric characteristics characteristic of thedirection of the alignment can be developed.

In another aspect of the present invention, there is provided a processfor producing the organic semiconductor structure, comprising the stepsof: allowing said liquid crystalline organic semiconductor material toexperience or be held at the liquid crystal development temperature ofthe liquid crystalline organic semiconductor material to once convertthe liquid crystalline organic semiconductor material to a liquidcrystal state; and cooling the liquid crystalline organic semiconductormaterial in a liquid crystal state to align and crystallize the liquidcrystalline organic semiconductor material.

According to the present invention, since the liquid crystalline organicsemiconductor material is a liquid crystalline material which is fluidat such a temperature that can maintain the liquid crystal state,coating of the liquid crystalline organic semiconductor material onto alayer forming face or the formation of a layer of the liquid crystallineorganic semiconductor material on a layer forming face, e.g., by vapordeposition followed by conversion of the state to a liquid crystal stateis easy. The organic semiconductor material in a liquid crystal statecan be then gradually cooled to form a defect-free organic semiconductorlayer in a crystal state which is uniform in charge transfercharacteristics and large in area. In the organic semiconductor layerthus formed, the molecular alignment is spontaneously realized by theself-organization of the liquid crystalline organic semiconductormaterial. The molecular alignment is similar to that of crystals. As aresult, excellent charge transfer characteristic as in molecularcrystals can be developed.

The organic semiconductor device in another aspect of the presentinvention includes a substrate, a gate electrode, a gate insulatinglayer, an organic semiconductor layer, a drain electrode, and a sourceelectrode. This organic semiconductor device is characterized in thatthe organic semiconductor layer comprises a liquid crystalline organicsemiconductor material having a core comprising L 6 π electron rings, M8 π electron rings, N 10 π electron rings, O 12 π electron rings, P 14 πelectron rings, Q 16 π electron rings, R 18 π electron rings, S 20 πelectron rings, T 22 π electron rings, U 24 π electron rings, and V 26 πelectron rings, wherein L, M, N, O, P, Q, R, S, T, U, and V are each aninteger of 0 (zero) to 6 and L+M+N+O+P+Q+R+S+T+U+V=1 to 6. In theorganic semiconductor device according to the present invention,functionalities and electric characteristics characteristic of alignmentdirection can be developed by specifying the direction of alignment ofthe liquid crystalline organic semiconductor material by means ofspecific means. The liquid crystalline organic semiconductor materialcan be aligned by forming an aligning layer on an organic semiconductormaterial layer forming face (hereinafter referred to as “layer formingface”; for example, the surface of a gate insulating layer), bysubjecting the organic semiconductor material layer forming face toalignment treatment, or by forming the organic semiconductor materiallayer so as to be brought into contact with a layer subjected toalignment treatment.

In the above organic semiconductor device according to the presentinvention, preferably, organic semiconductor molecules constituting theliquid crystalline organic semiconductor material are aligned in adirection orthogonal to the film thickness direction of a drainelectrode and a source electrode provided on the gate insulating layerand so as to be transversely arranged between the drain electrode andthe source electrode. Further, preferably, organic semiconductormolecules in the liquid crystalline organic semiconductor material arealigned in parallel with the film thickness direction of a drainelectrode and a source electrode provided on the gate insulating layer.According to the above constructions, liquid crystalline molecules inthe liquid crystalline organic semiconductor material can be aligned ina specific orientation or direction, and functionalities and electriccharacteristics characteristic of the direction of the alignment can bedeveloped.

In the organic semiconductor device according to the present invention,preferably, the organic semiconductor material has smectic liquidcrystallinity at a predetermined temperature below the heatdecomposition temperature of the organic semiconductor material and hasa charge mobility of not less than 10⁻⁵ cm²/V·s or a hole transportmobility of not less than 10⁻⁵ cm²/V·s.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the results of measurement ofphotoconductivity of an organic semiconductor layer formed by providingan organic semiconductor material on a substrate not subjected toalignment treatment;

FIG. 2 is a diagram showing the results of measurement ofphotoconductivity of an organic semiconductor layer formed by providingan organic semiconductor material on a substrate subjected to alignmenttreatment;

FIG. 3 is a cross-sectional view showing an embodiment of the organicsemiconductor device according to the present invention; and

FIG. 4 is a cross-sectional view showing another embodiment of theorganic semiconductor device according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

1. Organic Semiconductor Structure and Process for Producing the Same

The organic semiconductor structure according to the present inventionhas, at least a part thereof, an organic semiconductor layer formed ofan aligned liquid crystalline organic semiconductor material(hereinafter often referred to simply as “organic semiconductormaterial”). In the production process of an organic semiconductorstructure according to the present invention, the liquid crystallineorganic semiconductor material for constituting the organicsemiconductor layer is held at a temperature suitable for the conversionof the liquid crystalline organic semiconductor material to a liquidcrystal state and is then cooled to convert the liquid crystallineorganic semiconductor material to a crystal state. The contents of eachconstruction of the organic semiconductor structure and the productionprocess of the organic semiconductor structure will be described indetail.

Organic Semiconductor Layer

The organic semiconductor layer comprises an aligned liquid crystallineorganic semiconductor material. The organic semiconductor layer isformed on an aligning layer for placing molecular alignment in order ina liquid crystal phase, or on a substrate subjected to alignmenttreatment for that purpose, or in such a form that comes into contactwith a layer with an electrode and having an aligning ability. The layerwith an electrode and having an aligning ability may be a layer formedsubsequent to the formation of the organic semiconductor layer.

Preferably, the organic semiconductor material comprises an organiccompound having, in a part of the skeleton structure, L 6 π electronrings, M 8 π electron rings, N 10 π electron rings, O 12 π electronrings, P 14 π electron rings, Q 16 π electron rings, R 18 π electronrings, S 20 π electron rings, T 22 π electron rings, U 24 π electronrings, and V 26 π electron rings (also known as “core”), the skeletonstructure having a terminal structure (also known as “terminal group”)capable of developing liquid crystalline properties. The above structurecan realize the formation of an organic semiconductor layer which ischaracterized by developing a high level of charge transfercharacteristics derived from self-organization. Among materials havingthese molecular structures, those, which exhibit at least one liquidcrystal state at a temperature below the heat decomposition temperature,are applied. In the π electron rings constituting the organicsemiconductor material, L, M, N, O, P, Q, R, S, T, U, and V are each aninteger of 0 (zero) to 6, and L+M+N+O+P+Q+R+S+T+U+V=1 to 6.

In the liquid crystalline organic semiconductor material, 6 π electronrings include, for example, benzene, furan, thiophene, pyrrole,2H-pyran, 4H-thiopyran, pyridine, oxazole, isoxazole, thiazole,isothiazole, furazane, imidazole, pyrazole, pyrazine, pyrimidine, andpyridazine rings. 8 π electron rings include, for example, pentalene,indene, indolizine, and 4H-quinolizine rings. 10 π electron ringsinclude, for example, naphthalene, azulene, benzofuran, isobenzofuran,1-benzothiophene, 2-benzothiophene, indole, isoindole, 2H-chromene,1H-2-benzopyran, quinoline, isoquinoline, 1,8-naphthyridine,benzimidazole, 1H-indazole, benzoxazole, benzothiazole, quinoxaline,quinazoline, cinnoline, pteridine, purine, and phthalazine rings. 12 πelectron rings include, for example, heptalene, biphenylene,as-indacene, s-indacene, acenaphthylene, fluorene, and phenalene rings.14 π electron rings include, for example, phenanthrene, anthracene,carbazole, xanthene, acridine, phenanthridine, perimidine,1,10-phenanthroline, phenazine, phenarsazine, and tetrathiafulvalenerings. 16 π electron rings include, for example, fluoranthene,acephenanthrylene, aceanthrylene, pyrene, thianthrene, phenoxathiine,phenoxazine, and phenothiazine rings. 18 π electron rings include, forexample, triphenylene, chrysene, naphthacene, and pleiadene rings. 20 πelectron rings include, for example, perylene ring. 22 π electron ringsinclude, for example, picene, pentaphene, and pentacene rings. 24 πelectron rings include, for example, tetraphenylene and coronene rings.26 π electron rings include, for example, hexaphene, hexacene, andrubicene rings.

Skeleton structures having these aromatic rings in a part thereofinclude, for example, structures represented by chemical formulae 1 to34.

In the above formulae, R¹ and R² represent the following terminalstructure; R³'s, which may be the same or different, represent atrifluoromethyl group, an alkyl group, a nitro group, a halogen atom, ora hydrogen atom; X represents CH or N; and Y represents S or O.

A specific example of the terminal structure of R¹ and R² is one inwhich the rigid skeleton structure has on its one end any one of H (ahydrogen atom), a halogen atom, a cyano group, a nitro group, a hydroxylgroup and the like and on its other end a substituted or unsubstitutedalkyl group, a substituted or unsubstituted alkylthio group, asubstituted or unsubstituted alkoxyl group, a substituted orunsubstituted alkoxycarbonyl group, or a substituted or unsubstitutedalkylcarbonyl group. In this case, examples of substituents includehalogen atoms or cyano, sulfo, alkoxycarbonyl, alkoxy, hydroxy, aryloxy,acyloxy, aryl, and acyl groups.

Further, the terminal structure may be such that the skeleton structurehas on its both ends a substituted or unsubstituted alkyl group, asubstituted or unsubstituted alkylthio group, a substituted orunsubstituted alkoxyl group, a substituted or unsubstituted alkoxycarbonyl group, or a substituted or unsubstituted alkylcarbonyl group.In this case, examples of substituents include halogen atoms or cyano,sulfo, alkoxycarbonyl, alkoxy, hydroxy, aryloxy, acyloxy, aryl, and acylgroups.

This organic semiconductor material exhibits at least one type of liquidcrystal state at a temperature below the heat decomposition temperature.The expression “at a temperature below the heat decompositiontemperature” refers to such a state that the organic semiconductormaterial per se is not thermally decomposed. The heat decompositiontemperature varies depending upon organic semiconductor materialsapplied. Further, the expression “at least one type of liquid crystalstate” means that, even when an organic semiconductor material, whichexhibits a plurality of liquid crystal states, or an organicsemiconductor material having a non-liquid-crystal state is applied, amaterial, which exhibits at least one type of liquid crystal state, isused. For example, a smectic (hereinafter referred to also as “Sm”)liquid crystal, which will be described later, exhibits a plurality ofliquid crystal states of SmA phase, SmB phase, SmC phase, SmD phase . .. , and the material to be used should exhibit at least one of theseliquid crystal states.

The organic semiconductor material adopted in the organic semiconductorstructure according to the present invention is selected, from the abovespecific examples of organic semiconductor materials, by taking intoconsideration charge transfer characteristics and properties required ofthe organic semiconductor structure to be produced. A charge mobility ofnot less than 10⁻⁵ cm²/V·s may be mentioned as a property value used asa basis of selection.

Among the above various organic semiconductor materials, materialshaving a smectic liquid crystal phase are preferred. Liquid crystal is amaterial having a self-organizing function. In particular, for thesmectic liquid crystal, molecular alignment is spontaneously realized,and the liquid crystal is formed with a high orientation order as incrystals. An organic semiconductor layer formed of a material havingsmectic liquid crystallinity can exhibit excellent charge transferproperties as in molecular crystals. Further, when the degree oforientation order of the smectic liquid crystal constituting the organicsemiconductor layer is higher, molecules are aligned more orderly and ahigher level of orientation similar to that of crystals can be provided.Therefore, the intermolecular distance is very small. This leads to sucha significant effect that excellent charge transfer characteristicsderived from hopping conduction can be provided.

A specific example of a smectic liquid crystal is a liquid crystallineorganic semiconductor material having a phenylnaphthalene skeletonrepresented by chemical formula 35.

This compound is converted to a crystal state at a temperature of 54° C.or below. Therefore, this compound is held at 55 to 128° C., that is, atemperature capable of maintaining or experiencing a liquid crystalphase, and is then gradually cooled to 54° C. or below at a rate of 0.1to 1.0° C./min. As a result, this compound can be brought to a crystalstate while maintaining the well aligned state. The organicsemiconductor layer thus formed has a charge mobility of 10⁻⁵ to 5×10⁻²cm²/V·s, that is, excellent charge transfer characteristics.

Another example of a smectic liquid crystal is a liquid crystallineorganic semiconductor material having a phenylnaphthalene skeletonrepresented by chemical formula 36.

This compound is converted to a crystal state at a temperature of 79° C.or below. Therefore, this compound is heated to 80 to 121° C., that is,a temperature capable of maintaining or experiencing a liquid crystalphase, and is then gradually cooled to 79° C. or below at a rate of 0.1to 1.0° C./min. As a result, this compound can be brought to a crystalstate while maintaining the well aligned state. The organicsemiconductor layer thus formed has a charge mobility of 10⁻⁵ to 5×10⁻²cm²/V·s, that is, excellent charge transfer characteristics.

Still another example of a smectic liquid crystal is a liquidcrystalline organic semiconductor material having a terthiopheneskeleton represented by chemical formula 37.

This compound is converted to a crystal state at a temperature of 56° C.or below. Therefore, this compound is heated to 57 to 88° C., that is, atemperature capable of maintaining or experiencing a liquid crystalphase (SmF phase), and is then gradually cooled to 56° C. or below at arate of 0.1 to 1.0° C./min. As a result, this-compound can be brought toa crystal state while maintaining the well aligned state. The organicsemiconductor layer thus formed has a charge mobility of 10⁻⁵ to 5×10⁻²cm²/V·s, that is, excellent charge transfer characteristics.

A further example of a smectic liquid crystal is a liquid crystallineorganic semiconductor material having a terthiophene skeletonrepresented by chemical formula 38.

This compound is converted to a crystal state at a temperature of 54° C.or below. Therefore, this compound is heated to 55 to 84° C., that is, atemperature capable of maintaining or experiencing a liquid crystalphase (SmB phase), and is then gradually cooled to 54° C. or below at arate of 0.1 to 1.0° C./min. As a result, this compound can be brought toa crystal state while maintaining the well aligned state. The organicsemiconductor layer thus formed has a charge mobility of 10⁻⁶ to 5×10⁻³cm²/V·s, that is, excellent charge transfer characteristics.

A still further example of a smectic liquid crystal is a liquidcrystalline organic semiconductor material having a phenylbenzothiazoleskeleton represented by chemical formula 39.

This compound is converted to a crystal state at a temperature of 90° C.or below. Therefore, this compound is heated to 91 to 100° C., that is,a temperature capable of maintaining or experiencing a liquid crystalphase (SmA phase), and is then gradually cooled to 90° C. or below at arate of 0.1 to 1.0° C./min. As a result, this compound can be brought toa crystal state while maintaining the well aligned state. The organicsemiconductor layer thus formed has a charge mobility of 10⁻⁶ to 5×10⁻³cm²/V·s, that is, excellent charge transfer characteristics.

Further, materials suitable for the present invention may be selectedfrom materials described in Japanese Patent Laid-Open No. 316442/1997and U.S. Pat. No. 5,766,510.

The above liquid crystalline organic semiconductor materials arecharacterized in that any charge of electrons and holes can betransported, that is, “transfer of both polarities” is possible, andthat the dependence of the charge mobility upon field strength andtemperature is small in the case of an identical liquid crystal phasestate.

In the organic semiconductor structure according to the presentinvention, depending upon the type of the liquid crystalline organicsemiconductor material used in the formation of the organicsemiconductor layer, the relationship between the temperature, at whichthe liquid crystalline organic semiconductor material is converted to acrystal phase, and the temperature, at which the liquid crystal phasecan be maintained or experienced, is taken into consideration. Based onthis consideration, an organic semiconductor material is formed at sucha temperature that a liquid crystal phase is maintained or experienced.Thereafter, the organic semiconductor material is gradually cooled at arate of 0.1 to 1.0° C./min to a temperature below the crystal phaseformation temperature. In this case, the temperature at which the liquidcrystalline organic semiconductor material can maintain or experiencethe liquid crystal phase varies depending upon the type of the liquidcrystalline organic semiconductor material. In the present invention,however, any liquid crystalline organic semiconductor material, whichexhibits at least one type of liquid crystal state at a temperaturebelow the heat decomposition temperature thereof, can be used.

When the rate of gradual cooling is less than 0.1° C./min,disadvantageously, the time necessary for cooling is too long. On theother hand, when the rate of gradual cooling exceeds 1.0° C./min,structure defects with respect to charge transfer disadvantageouslyoccur due to rapid contraction in volume of the crystal phase.

Methods for forming an organic semiconductor material on a layer formingface at such a temperature that a liquid crystal phase (a phase in aliquid crystal state) can be maintained or experienced include onewherein an organic semiconductor material is coated in a liquid crystalstate on the layer forming face and the coating is then gradually cooledand converted to a crystal state, and one wherein an organicsemiconductor material is vapor-deposited, e.g., by PVC or CVD, on thelayer forming face, and the deposit is heated to a temperature at whicha liquid crystal phase is exhibited to once experience the liquidcrystal phase, followed by gradual cooling to covert the deposit to acrystal state. In particular, the above organic semiconductor materialis fluid at such a temperature that the liquid crystal state ismaintained. Therefore, the organic semiconductor material can be coatedin this liquid crystal state to form a coating which is then graduallycooled to convert the coating to a crystal state. According to thismethod, a large-area organic semiconductor layer having uniform chargetransfer characteristics can be very easily formed. Various coating andprinting methods may be used for coating. The term “crystal phase” or“crystal state” as used herein means that the liquid crystalline organicsemiconductor material is in the aggregate formed at a temperature belowthe liquid crystal-crystal phase transition temperature.

In the liquid crystalline organic semiconductor material in a crystalstate, the liquid crystalline molecules are aligned in a givenorientation or direction by aligning means which will be descried later.Therefore, the liquid crystalline molecules are regularly properlyaligned as in molecular crystals, and the average intermoleculardistance of the liquid crystalline molecules is as small as 0.3 to 0.4nm. In the organic semiconductor layer in a crystal state with thisintermolecular distance, electron correlation between moleculesthemselves is very strong. In this case, advantageously, hoppingprobability of carriers is high, and a high level of charge transfercharacteristics can be realized. For example, in the liquid crystallinemolecules having smectic liquid crystallinity represented by chemicalformula 35, when the average intermolecular distance is 0.3 to 0.4 nm, ahigh level of charge transfer characteristics, that is, a chargemobility as high as 10⁻³ to 10⁻² cm²/V·s, can be realized.

Liquid Crystal Aligning Means

In the organic semiconductor structure according to the presentinvention, the organic semiconductor layer is provided in contact with aliquid crystal aligning layer. In this case, the organic semiconductorlayer is formed of a liquid crystalline organic semiconductor materialprovided in contact with the liquid crystal aligning layer toanisotropically align the liquid crystalline organic semiconductormaterial in a specific direction. Specifically, an aligning layer as theliquid crystal aligning means is provided so as to come into contactwith the organic semiconductor layer, whereby the liquid crystallineorganic semiconductor material is anisotropically aligned by thealigning layer. In the alignment of the liquid crystalline organicsemiconductor material using the liquid crystal aligning means, evenwhen the material is converted by cooling from a liquid crystal state toa crystal phase, crystallization can be carried out while maintainingthe molecular alignment. Each crystal domain in crystals formed in thisway is so large that the charge mobility is advantageously increased.

Examples of the aligning means include the formation of a liquid crystalaligning layer on a liquid crystalline organic semiconductor materialforming face (a layer forming face, for example, a surface of a gateinsulating layer which will be described later), aligning treatment suchas rubbing treatment, and contact with a layer subjected to alignmenttreatment. These aligning means can align liquid crystalline moleculesof the liquid crystalline organic semiconductor material in a specificorientation or direction. Therefore, functionalities and electriccharacteristics characteristic of the orientation or direction of thealignment can be developed.

The relationship between the aligning means and the liquid crystallineorganic semiconductor material is preferably such that the liquidcrystalline organic semiconductor material is stacked and aligned on aliquid crystal aligning layer formed of a polyimide material, that theliquid crystalline organic semiconductor material is stacked and alignedon a liquid crystal aligning layer formed of a cured resin having fineconcaves and convexes on its surface, or that the liquid crystallineorganic semiconductor material is provided on a substrate formed of acured resin having fine concaves and convexes on its surface.

Various liquid crystal aligning layers can be applied for liquid crystalalignment purposes. In the organic semiconductor structure according tothe present invention, however, the liquid crystal aligning layer ispreferably any one of a layer formed by coating a polyimide material andthen subjecting the coating to rubbing treatment, a layer formed of acured resin having fine concaves and convexes, and a layer which isformed of a cured resin having fine concaves and convexes and functionsalso as a substrate. The alignment of liquid crystalline molecules byexternal field such as electric field or magnetic field is alsopossible.

A typical liquid crystal aligning layer is a layer formed by coating apolyimide resin and then subjecting the coating to rubbing treatment. Inaddition to the polyimide resin, other materials usable herein includeresin materials such as acrylic, polychloropyrene, polyethyleneterephthalate, polyoxymethylene, polyvinyl chloride, poly(vinylidenefluoride), cyanoethyl pullulan, polymethyl methacrylate, polysulfone,polycarbonate, and polyimide resins. These materials can be classifiedinto types which can perpendicularly align the liquid crystal and typeswhich can horizontally align the liquid crystal. Specific examples ofcoating methods include spin coating, casting, and pull-up methods. Theliquid crystal aligning layer may be provided between the substrate andthe organic semiconductor layer or on an overcoat overlying the organicsemiconductor layer.

The liquid crystal aligning layer formed of a cured resin having fineconcaves and convexes may be formed, for example, by forming a layer ofa curable resin, either subjecting the surface of the layer to rubbingtreatment to form concaves and convexes, or pressing a shaping membercapable of forming fine concaves and convexes against the surface of theuncured curable resin layer, and then curing the resin layer. Thesurface of the cured resin has fine concaves and convexes inpredetermined orientation which can align liquid crystalline moleculesof the liquid crystalline organic semiconductor material in thisorientation. Curable resins include ultraviolet-curable acrylic resinsand ultraviolet-curable fluorocarbon resins. At that time, particularlypreferably, the liquid crystal aligning layer formed of a cured resinhaving fine concaves and convexes is integral with a substrate.

The fine concaves and convexes comprise fine grooves provided in anidentical direction. In the grooves in the concave-convex part, thedepth is approximately 0.01 to 1.0 μm, preferably 0.03 to 0.3 μm, thewidth is approximately 0.05 to 1.0 μm, and the pitch of adjacent groovesis approximately 0.1 to 2.0 μm. When the depth of the grooves is lessthan 0.01 μm, the liquid crystal molecules cannot be properly aligned.On the other hand, when the depth of the grooves exceeds 1.0 μm, thealignment of the liquid crystal is sometimes disordered at the edge ofthe grooves. When the width of the grooves is less than 0.05 μm, thepreparation of the grooves is difficult. On the other hand, when thewidth of the grooves exceeds 1.0 μm, the alignment force at the centerof the groove is sometimes lowered. When the pitch of the grooves isless than 0.1 μm, the preparation of the grooves is difficult. On theother hand, when the pitch of the grooves exceeds 2.0 μm, the disorderof the alignment of the liquid crystal is likely to occur.

In the organic semiconductor structure according to the presentinvention, a first embodiment of the liquid crystal aligning layercomprises a substrate, a liquid crystal aligning layer, and an organicsemiconductor layer successively stacked in that order. A secondembodiment of the liquid crystal aligning layer comprises a substrate,an organic semiconductor layer, and a liquid crystal aligning layersuccessively stacked in that order. A third embodiment of the liquidcrystal aligning layer comprises a substrate, a liquid crystal aligninglayer, an organic semiconductor layer, and a liquid crystal aligninglayer successively stacked in that order. Thus, in the presentinvention, when the organic semiconductor layer is constructed incontact with a layer subjected to alignment treatment, a high level ofalignment of the liquid crystalline molecules constituting the liquidcrystalline organic semiconductor material can be realized.

Organic Semiconductor Structure

The organic semiconductor structure having the above construction willbe described in more detail with reference to FIGS. 1 and 2.

FIG. 1 is a diagram showing the results of measurement ofphotoconductivity of an organic semiconductor layer formed by providingan organic semiconductor material having a phenylnaphthalene skeleton ona substrate not subjected to alignment treatment. FIG. 1(A) shows theresults of measurement of photoconductivity for a liquid crystal phase(90° C., SmB phase), and FIG. 1(B) shows the results of measurement ofphotoconductivity for a crystal phase (65° C.). When no alignmenttreatment is carried out, substantially no photoconductivity is provideddue to the influence of grain boundaries (structure defects) in thecrystal phase.

FIG. 2 is a diagram showing the results of measurement ofphotoconductivity of an organic semiconductor layer formed by providingan organic semiconductor material having a phenylnaphthalene skeleton ona substrate, bringing the organic semiconductor material to a liquidcrystal state, then gradually cooling the organic semiconductor materialto perform alignment treatment. For the material subjected to alignmenttreatment, even in a crystal state, a high level of photoconductivitycould be provided. Further, in this case, the photocurrent is largerthan that of the smectic layer. In FIG. 2, ISO represents an isotropicphase, SmA a smectic A phase, SmB a smectic B phase, and K a crystalphase.

The reason why this phenomenon takes place is believed to be as follows.Specifically, it is known that the charge transfer characteristics of arodlike liquid crystalline organic semiconductor material is notinfluenced by the grain structure of the smectic liquid crystal state.Crystallization without alignment treatment probably causes the grainstructure to be changed to a fine crystal state. This change results inthe occurrence of detects among crystals which inhibits charge transfer.On the other hand, for example, in the case of an organic semiconductorlayer in a smectic liquid crystal state in contact with an aligningtreatment layer (same as the liquid crystal aligning layer), in additionto an adjacent field, a structural order over a long distanceattributable to the aligning treatment layer exists. During theconversion of the liquid crystal state to the crystal state, thelong-distance order is maintained. Therefore, it is considered that theaggregates of fine polycrystals are not formed. By virtue of this, evenin the crystal state, a high level of mobility is provided, and, in sometemperature region, the mobility is higher than that in the smecticliquid crystal state.

The crystal phase of the liquid crystalline rodlike organicsemiconductor material has a high level of charge mobility, and anorganic semiconductor structure utilizing this property has highperformance. Organic semiconductor devices comprising this organicsemiconductor structure, for example, transistors, organic ELs, solarcells, and sensors, have unprecedentedly high performance. Inparticular, when this organic semiconductor structure is utilized in anorganic EL element, a high level of polarized light emission derivedfrom the molecular alignment can be realized.

Liquid crystalline rodlike organic semiconductor materials per seexhibit electroluminescence. However, the introduction of a side chaininto an organic material, which can function as a luminescence center,followed by the addition of the treated organic material to a liquidcrystalline organic semiconductor as a base material can control thepositional relationship between a charge transfer path for the organicsemiconductor and the organic material as the luminescence center. Thiscan contribute to improved emission efficiency.

Examples of contemplated constructions of organic ELs using the aboveorganic semiconductor material are as follows:

(i) substrate/electrode/liquid crystal aligning layer/liquid crystallineorganic semiconductor layer/electrode,

(ii) substrate/electrode/liquid crystalline organic semiconductorlayer/liquid crystal aligning layer/electrode/substrate,

(iii) substrate/liquid crystal aligning layer/electrode/liquidcrystalline organic semiconductor layer/electrode,

(iv) substrate/electrode/liquid crystal aligning layer/liquidcrystalline organic semiconductor layer/liquid crystal aligninglayer/electrode/substrate, and

(v) substrate/electrode/liquid crystal aligning layer/electrode/liquidcrystal aligning layer/substrate.

2. Organic Semiconductor Device

As shown in FIGS. 3 and 4, an organic semiconductor device 101 accordingto the present invention includes at least a substrate 11, a gateelectrode 12, a gate insulating layer 13, an organic semiconductor layer14 comprising an aligned liquid crystalline organic semiconductormaterial, a drain electrode 15, and a source electrode 16. In thisorganic semiconductor device 101, the organic semiconductor layer 14 isformed of an organic semiconductor material constituting the organicsemiconductor structure according to the present invention.

Examples of constructions include an inversely staggered structurecomprising the substrate 11 and, provided on the substrate 11 in thefollowing order, the gate electrode 12, the gate insulating layer 13,the organic semiconductor layer 14 formed of an aligned liquidcrystalline organic semiconductor material, the drain electrode 15 andthe source electrode 16, and a protective film (not shown), and acoplanar structure comprising the substrate 11 and, provided on thesubstrate 11 in the following order, the gate electrode 12, the gateinsulating layer 13, the drain electrode 15 and the source electrode 16,the organic semiconductor layer 14 formed of an aligned liquidcrystalline organic semiconductor material, and a protective film (notshown). The organic semiconductor device 101 having the aboveconstruction is operated in a storage or deficiency state according tothe polarity of voltage applied to the gate electrode 12. The membersconstituting the organic semiconductor device will be described indetail.

Substrate

The substrate 11 may be selected from a wide range of insulatingmaterials. Examples of such materials include various insulatingmaterials, for example, inorganic materials such as glasses and aluminasinters, and polyimide films, polyester films, polyethylene films,polyphenylene sulfide films, and polyparaxylene films. In particular,the use of films of polymeric compounds is very useful becauselightweight and flexible organic semiconductor devices can be prepared.The thickness of the substrate 11 applied in the present invention isabout 25 μm to 1.5 mm.

Gate Electrode

The gate electrode 12 is preferably an electrode formed of an organicmaterial such as polyaniline or polythiophene, or an electrode formed bycoating an electrically conductive ink. Since these electrodes can beformed by coating an organic material or an electrically conductive ink,the electrode formation process is advantageously very simple. Specificexamples of coating methods include spin coating, casting, and pull-upmethods.

When a metallic film is formed as an electrode, a conventional vacuumfilm formation method can be used. Specifically, a mask film formationmethod or a photolithographic method may be used. In this case,materials for electrode formation usable herein include inorganicmaterials, for example, metals such as gold, platinum, chromium,palladium, aluminum, indium, molybdenum, and nickel, alloys using thesemetals, polysilicon, amorphous silicon, tin oxide, indium oxide, andindium tin oxide (ITO). Two or more of these materials may be used incombination.

The film thickness of the gate electrode is preferably about 50 to 1000nm, although the film thickness varies depending upon the electricalconductivity of the material. The lower limit of the thickness of thegate electrode varies depending upon the electrical conductivity of theelectrode material and the strength of adhesion between the gateelectrode and the underlying substrate. The upper limit of the thicknessof the gate electrode should be such that, when a gate insulating layerand a source-drain electrode pair which will be described later isprovided, the insulation covering by the gate insulating layer in alevel difference part between the underlying substrate and the gateelectrode is satisfactory and an overlying electrode pattern is notbroken. In particular, when a flexible substrate is used, a stressbalance should be taken into consideration.

Gate Insulating Layer

As with the gate electrode 12, the gate insulating layer 13 ispreferably formed by coating an organic material. Organic materialsusable herein include polychloropyrene, polyethylene terephthalate,polyoxymethylene, polyvinyl chloride, poly(vinylidene fluoride),cyanoethyl pullulan, polymethyl methacrylate, polysulfone,polycarbonate, and polyimide. Specific examples of coating methodsinclude spin coating, casting, and pull-up methods.

Alternatively, the gate insulating layer may be formed by a conventionalpattern process such as CVD. In this case, inorganic materials such asSiO₂, SiNx, and Al₂O₃ are preferably used. Two or more of thesematerials may be used in combination.

Since the mobility of the organic semiconductor device depends uponfield strength, the thickness of the gate insulating layer is preferablyabout 50 to 300 nm. In this case, the withstand voltage is preferablynot less than 2 MV/cm.

Drain Electrode and Source Electrode

The drain electrode 15 and the source electrode 16 are preferably formedof a metal having a large work function. The reason for this is asfollows. Specifically, in the liquid crystalline organic semiconductormaterial which will be described later, carriers for transportingcharges are holes, and, hence, the liquid crystalline organicsemiconductor material should be in ohmic contact with the organicsemiconductor layer 14. The term “work function” as used herein refersto a potential difference necessary for taking electrons out of thesolid to the outside of the solid, and the work function is defined as avalue obtained by dividing a difference in energy, between vacuum leveland Fermi level, by the quantity of electric charge. The work functionis preferably about 4.6 to 5.2 eV, and specific examples of such metalsinclude gold, platinum, and transparent electrically conductive films(for example, indium tin oxide and indium zinc oxide). The transparentelectrically conductive film may be formed by sputtering or electronbeam (EB) evaporation.

The thickness of the drain electrode 15 and the source electrode 16applied in the present invention is about 50 to 100 nm.

Organic Semiconductor Layer

The organic semiconductor layer 14 is formed of an aligned liquidcrystalline organic semiconductor material and may be formed, forexample, by subjecting the liquid crystalline organic semiconductormaterial to the above alignment treatment and then converting thealigned liquid crystalline organic semiconductor material to a crystalstate. Specific examples of organic semiconductor materials are asdescribed above.

Further, the alignment treatment of the organic semiconductor material,the liquid crystal aligning layer, etc. are also as described above.Specifically, in the present invention, since the liquid crystallineorganic semiconductor material is used, upon the above alignmenttreatment, liquid crystalline molecules are aligned in a givenorientation or direction. As compared with conventional organicsemiconductor layers, the organic semiconductor layer 14, which has beensubjected to alignment treatment and converted to a crystal phase, hasexcellent effects, that is, is free from cracking or the like and doesnot cause any harmful effects such as lowered charge transport speedbased on the cracking.

Embodiments of the alignment of liquid crystalline molecules include (i)an embodiment as shown in FIG. 3 in which the liquid crystallinemolecules are aligned in a direction orthogonal to the film thicknessdirection of the drain electrode 15 and the source electrode 16 providedon the gate insulating layer 13 and so as to be transversely arrangedbetween the drain electrode 15 and the source electrode 16, and (ii) anembodiment as shown in FIG. 4 in which the liquid crystalline moleculesare aligned in parallel with the film thickness direction of the drainelectrode 15 and the source electrode 16 provided on the gate insulatinglayer 13.

In the organic semiconductor layer 14 in the embodiments shown in FIGS.3 and 4, as described above, adjacent rodlike liquid crystallinemolecules are aligned in an intermolecular distance of about 0.3 to 0.4nm. Therefore, for example, in the case of SmA phase in a smectic liquidcrystal system, large anisotropy of charge transport characteristicsappears between the Sm layer in its in-plane direction having a smallerintermolecular distance and the Sm layer in its inter-plane direction,perpendicular-to the Sm layer, having a large intermolecular distance ofnot less than 3 nm. The anisotropy of the charge transportcharacteristics is based on the fact that molecular alignment isspontaneously realized by the self-organization of the liquidcrystalline organic semiconductor material and the liquid crystal isformed with a high orientation order as in crystals.

The organic semiconductor layer thus formed has such a unique effectthat a defect-free, uniform, large-size organic semiconductor layer canbe formed. The electron transport speed of the organic semiconductorlayer 14 is not less than 10⁻² cm²/V·s, and, more preferably, the holetransport speed is not less than 10⁻² cm²/V·s. In order to provide suchproperty values, the liquid crystalline organic semiconductor material,the alignment treatment, etc. may be examined at any time before theformation of the organic semiconductor layer. This charge transportspeed of the organic semiconductor layer 14 can advantageouslycontribute, for example, to a lowering in drive voltage of organicthin-film transistors or to an improvement in response speed.

When the layer forming face, on which the liquid crystalline organicsemiconductor material is formed, is a gate insulating layer or asubstrate, the gate insulating layer or the substrate may be subjectedto rubbing treatment so that the gate insulating layer or the substrateserves also as the alignment treatment film.

The thickness of the alignment control layer is preferably in such athickness range that does not hinder ohmic contact of the drainelectrode 15 and the source electrode 16 with the organic semiconductorlayer 14, that is, in the range of 0.5 to 10 nm.

Interlayer Insulating Layer

An interlayer insulating layer is preferably provided in the organicsemiconductor device 101. The interlayer insulating layer is providedfrom the viewpoint of preventing contamination of the surface of thegate electrode 12 in forming the drain electrode 15 and the sourceelectrode 16 on the gate insulating layer 13. Therefore, the interlayerinsulating layer is formed on the gate insulating layer 13 before theformation of the drain electrode 15 and the source electrode 16. Afterthe formation of the source electrode 15 and the drain electrode 16, theinterlayer insulating layer in its part located above the channel regionis entirely or partly removed. Preferably, the interlayer insulatinglayer in its region to be removed has a size equal to the size of thegate electrode 12.

Materials usable for constituting the interlayer insulating layerinclude inorganic materials such as SiO, SiNx, Al₂O₃ and organicmaterials such as polychloropyrene, polyethylene terephthalate,polyoxymethylene, polyvinyl chloride, poly(vinylidene fluoride),cyanoethyl pullulan, polymethyl methacrylate, polysulfone,polycarbonate, and polyimide.

Organic Semiconductor Device

Constructions adoptable in the organic semiconductor device according tothe present invention include

(i) substrate/gate electrode/gate insulating layer (which serves also asliquid crystal aligning layer)/source-drain electrode/liquid crystallineorganic semiconductor layer/(protective layer),

(ii) substrate/gate electrode/gate insulating layer/source-drainelectrode/liquid crystal aligning layer/liquid crystalline organicsemiconductor layer/(protective layer),

(iii) substrate/gate electrode/gate insulating layer (which serves alsoas liquid crystal aligning layer)/liquid crystalline organicsemiconductor layer/source-drain electrode/(protective layer),

(iv) substrate/gate electrode/gate insulating layer (which serves alsoas liquid crystal aligning layer)/liquid crystalline organicsemiconductor layer/substrate with source-drain electrode patternedthereon (which serves also as protective layer),

(v) substrate/source-drain electrode/liquid crystalline organicsemiconductor layer/gate insulating layer (which serves also as liquidcrystal aligning layer)/gate electrode/substrate (which serves also asprotective layer), and

(vi) substrate (which serves also as aligning layer)/source-drainelectrode/liquid crystalline organic semiconductor layer/gate insulatinglayer/gate electrode/substrate (which serves also as protective layer).

In the above organic semiconductor device, after the formation of a cellstructure, pouring of a liquid crystal into the cell by utilizingcapillarity followed by gradual cooling to crystallize the liquidcrystalline organic semiconductor material is a simple productionprocess. Alternatively, however, the organic semiconductor material maybe vapor-deposited or coated.

EXAMPLES

The following Examples further illustrate the present invention.

An organic semiconductor device having a construction of substrate/gateelectrode/gate insulating layer (serving also as liquid crystal aligninglayer)/source-drain electrode/liquid crystalline organic semiconductorlayer (/protective layer) was formed. In this Example, a layer of aliquid crystalline organic semiconductor material was formed on a glasssubstrate. The layer was then heated to a liquid crystal phase, therebybringing the layer to a liquid crystal state. The layer was thengradually cooled to a crystal phase.

Substrate

A glass substrate (thickness 1.1 mm, Corning 1737), which had beenultrasonically cleaned with a neutral detergent, pure water, acetone,and IPA in that order, was used as a substrate.

Gate Electrode

A strip pattern (electrode width 100 μm, electrode spacing 5 mm) of gold(Au) (thickness 300 nm) was formed on the substrate through a metallicmask by resistance heating type vapor deposition. Thus, a gate electrodewas formed. The same electrode pattern can be formed by patterning anITO electrode by a wet process.

Gate Insulating Layer and its Alignment Treatment

(a) Where molecules constituting the liquid crystalline organicsemiconductor material were aligned horizontally to the substrate (seeFIG. 3):

A gate insulating layer was formed as follows. A photosensitivepolyimide (a dilution of 10 g of UR-3140 (Toray Industries, Inc.) with25 g of n-methylpyrrolidone) was spin-coated. The coating was dried at100° C. The dried coating was exposed and developed to expose a gateelectrode terminal. Thereafter, the layer was fired at a maximumtemperature of 350° C. to form a 300 nm-thick gate insulating layer.

The surface of the polyimide film thus formed was subjected to alignmenttreatment by rubbing with rubbing cloth, which was a polyester woundaround a 48-mm roller, under conditions of 1200 rpm and substrate movingspeed 600 mm/min. The rubbing was carried out in both a directionparallel to a channel-length direction (a charge transport direction)and a direction perpendicular to the channel-length direction.

In this Example, upon the alignment treatment, the liquid crystallineorganic semiconductor material used in this Example is horizontallyaligned. Therefore, the effect of the anisotropy of charge transfer(difference in charge transfer characteristics between major axisdirection and minor axis direction of molecules) derived from thedirection of the liquid crystal alignment on the characteristics of TFTwas compared.

(b) Where molecules constituting the liquid crystalline organicsemiconductor material were aligned perpendicularly to the substrate(see FIG. 4):

A 100 nm-thick SiO₂ film was formed on the substrate with the gateelectrode provided thereon by RF sputtering (output 100 W×30 min). Theliquid crystalline organic semiconductor material used in this Exampleis aligned perpendicularly on this substrate. Therefore, when TFT isconstructed, charges are dominantly transported in a directionperpendicular to the major axis of molecules. In this case, the chargetransfer is the same as that in the above case where rubbing has beencarried out in a direction which is a horizontal alignment direction andperpendicular to the channel-length direction. In this Example, theeffect of a liquid crystal side chain structure on charge injection andthe effect of the liquid crystal side chain structure on thecharacteristics of TFT were investigated.

Source-Drain Electrode

Gold was deposited by resistance heating type vapor deposition through ametallic mask to form a source-drain electrode pad (channel length 50μm, channel width 4 mm) (electrode thickness 100 nm). Aluminum (Al) wasused as an electrode wire which was led out from the source-drainelectrode pad.

Liquid Crystalline Organic Semiconductor Layer

A liquid crystalline organic semiconductor material using 6-TTP-12represented by chemical formula 37 was deposited by resistance heatingtype vapor deposition through a metallic mask so that a rectangularpattern of 4 mm×100 μm was formed in a channel form between theabove-formed source and drain to form a 50 nm-thick organicsemiconductor layer which was then heated to 60° C. for transition to aliquid crystal phase. Thereafter, the layer was cooled at a rate of 0.1°C./min to form a functional film having a crystal phase which maintainsa good aligned state by virtue of the aligned state of the liquidcrystal phase.

Evaluation of Characteristics

(a) Where molecules constituting liquid crystalline organicsemiconductor material were aligned horizontally to substrate (see FIG.3), that is, in the case of an organic semiconductor element wherein a300 nm-thick polyimide layer was used as a gate insulating layer, asource electrode and a drain electrode were formed with a channel lengthof 50 μm and a channel width of 4 mm and 6-TTP-12 (compound 37) was usedas an organic semiconductor layer:

For this type of an organic semiconductor element, an element in a finepolycrystalline state just after the preparation (Comparative Example 1)and an element prepared by heating the material in the finepolycrystalline state to 60° C. for phase transition to a liquid crystalphase and then gradually cooling the material at a rate of 0.1° C./minto form a crystal phase (Example 1) were provided. For each of theelements, a source-to-drain voltage Vds: 0 to −30 V and a gate voltageVg: 0 to −30 V were applied, and the behavior of a change insource-to-drain current Ids upon the application of gate voltage Vg wasdetermined and evaluated.

The element of Comparative Example 1 was not prepared through a liquidcrystal phase. In this element, in the above measurement voltage range,there was no significant change in drain current depending upon the gatevoltage.

On the other hand, the element of Example 1 is an element having acrystal phase formed through a liquid crystal phase. In this element, inthe above measurement range, under given Vds conditions, the draincurrent increased with increasing the absolute value of the gatevoltage. Under constant gate voltage conditions, the drain currentincreased with increasing the absolute value of the source-to-drainapplied voltage, and further increasing the absolute value of thesource-to-drain applied voltage resulted in such a phenomenon that thedrain current in the so-called “organic transistor” reached a saturationregion.

This tendency was particularly significant for the element in which thesurface of the gate insulating layer before the formation of the sourceelectrode and the drain electrode was rubbed in “a channel-widthdirection” (a direction perpendicular to the channel-length direction).In this case, the charge mobility of the organic semiconductor layer was2×10⁻³ cm²/V·s based on Vg and Vds in the saturation region of the draincurrent and the structure of the element of Example 1.

The charge mobility of the organic semiconductor material (6-TTP-12represented by chemical formula 37) was measured by TOF for a counterelectrode-substrate pair (sandwich type) sample construction. As aresult, the charge mobility corresponding to the phase series was asfollows: crystal phase (immeasurable)/56° C./SmF phase (5×10⁻³cm²/V·s)/88° C./isotropic phase (approximately 10⁻⁵ cm²/V·s).

As is apparent from the above results, for the element of ComparativeExample 1 corresponding to the conventional method, despite the crystalphase, a high-speed charge transfer phenomenon and good transistorcharacteristics were not provided. On the other hand, the element of theexample of the present invention had good charge mobility and could berealized in a very simpler manner than the conventional organictransistor having high mobility.

On the other hand, for the element in which the surface of the gateinsulating layer before the formation of the source electrode and thedrain electrode was rubbed in “a channel-length direction” (a directionparallel to the channel-length direction), the charge mobility of theorganic semiconductor layer was not more than 5×10⁻⁵ cm²/V·s based on Vgand Vds in the saturation region of the drain current, and the structureof the element of Example 1. This value was smaller than that in theelement which had been rubbed in the above “channel-width direction.”The reason for this is believed to be as follows. The hopping siteinvolved in the charge transfer of the liquid crystalline organicsemiconductor material used is overlap of a conjugated system in askeleton structure within the molecule of the material, whereas thealkyl chain as a terminal group used in this example serves as aninsulating layer between molecular arrangements in the skeletonstructure as a conductive path. By virtue of this, it is considered thatthe anisotropy of conductive characteristics has been realized by anidentical material according to the molecular arrangement direction ofthe organic semiconductor.

(b) Where molecules constituting liquid crystalline organicsemiconductor material were aligned perpendicularly to substrate (seeFIG. 4), that is, in the case of an organic semiconductor elementwherein 100 nm-thick SiO₂ was used as a gate insulating layer, a sourceelectrode and a drain electrode were formed with a channel length of 50μm and a channel width of 4 mm and 6-TTP-12 (compound 37) was used as anorganic semiconductor layer:

For this type of an organic semiconductor element, an element in a finepolycrystalline state just after the preparation (Comparative Example 2)and an element prepared by heating the material in the finepolycrystalline state to 60° C. for phase transition to a liquid crystalphase and then gradually cooling the material at a rate of 0.1° C./minto form a crystal phase (Example 2) were provided. For each of theelements, a source-to-drain voltage Vds: 0 to −30 V and a gate voltageVg: 0 to −30 V were applied, and the behavior of a change insource-to-drain current Ids upon the application of gate voltage Vg wasdetermined and evaluated.

Also in (b), as with the above-described (a), for the element ofComparative Example 2 which had not been prepared through a liquidcrystal phase, in the above measurement voltage range, there was nosignificant change in drain current depending upon the gate voltage.

On the other hand, for the element of Example 2 which is an elementhaving a crystal phase formed through a liquid crystal phase, as withthe above (a), transistor characteristics were developed, and the chargemobility of the organic semiconductor layer was 2 to 4×10⁻⁴ cm²/V·s,demonstrating that, according to the present invention, an organictransistor can be easily realized.

1. An organic semiconductor structure having, in at least a partthereof, an organic semiconductor layer comprising an aligned liquidcrystalline organic semiconductor material, said liquid crystallineorganic semiconductor material comprising an organic compound having acore comprising L 6 π electron rings, M 8 π electron rings, N 10 πelectron rings, O 12 π electron rings, P 14 π electron rings, Q 16 πelectron rings, R 18 π electron rings, S 20 π electron rings, T 22 πelectron rings, U 24 π electron rings, and V 26 π electron rings,wherein L, M, N, O, P, Q, R, S, T, U, and V are each an integer of 0(zero) to 6 and L+M+N+O+P+Q+R+S+T+U+V=1 to 6, said liquid crystallineorganic semiconductor material exhibiting at least one liquid crystalstate at a temperature below the heat decomposition temperature thereof.2. An organic semiconductor structure having, in at least a partthereof, an organic semiconductor layer comprising an aligned liquidcrystalline organic semiconductor material, said liquid crystallineorganic semiconductor material comprising an organic compound having acore comprising L 6 π electron rings, M 8 π electron rings, N 10 πelectron rings, O 12 π electron rings, P 14 π electron rings, Q 16 πelectron rings, R 18 π electron rings, S 20 π electron rings, T 22 πelectron rings, U 24 π electron rings, and V 26 π electron rings,wherein L, M, N, O, P, Q, R, S, T, U, and V are each an integer of 0(zero) to 6 and L+M+N+O+P+Q+R+S+T+U+V=1 to 6, said liquid crystallineorganic semiconductor material exhibiting at least a smectic liquidcrystal phase state at a temperature below the heat decompositiontemperature thereof.
 3. An organic semiconductor structure having, in atleast a part thereof, an organic semiconductor layer comprising analigned liquid crystalline organic semiconductor material, said liquidcrystalline organic semiconductor material comprising an organiccompound having a core comprising L 6 π electron rings, M 8 π electronrings, N 10 π electron rings, O 12 π electron rings, P 14 π electronrings, Q 16 π electron rings, R 18 π electron rings, S 20 π electronrings, T 22 π electron rings, U 24 π electron rings, and V 26 π electronrings, wherein L, M, N, O, P, Q, R, S, T, U, and V are each an integerof 0 (zero) to 6 and L+M+N+O+P+Q+R+S+T+U+V=1 to 6, said liquidcrystalline organic semiconductor material having, at its both ends, aterminal group capable of developing liquid crystallinity.
 4. Theorganic semiconductor structure according to any one of claims 1 to 3,wherein at least a part of said liquid crystalline organic semiconductormaterial in the organic semiconductor layer has been aligned andcrystallized by holding the liquid crystalline organic semiconductormaterial at a temperature suitable for the conversion of the liquidcrystalline organic semiconductor material to a liquid crystal state,and then cooling the liquid crystalline organic semiconductor material.5. The organic semiconductor structure according to any one of claims 1to 4, wherein said organic semiconductor layer is provided in contactwith a liquid crystal aligning layer and the provision of the organicsemiconductor layer in contact with the liquid crystal aligning layerallows the liquid crystalline organic semiconductor material to beanisotropically aligned in a specific direction.
 6. The organicsemiconductor structure according to claim 5, wherein said liquidcrystal aligning layer is formed of a polyimide material.
 7. The organicsemiconductor structure according to claim 5, wherein said liquidcrystal aligning layer is formed of a cured resin having fine concavesand convexes on its surface.
 8. The organic semiconductor structureaccording to claim 5, wherein said liquid crystal aligning layercomprises a cured resin which has fine concaves and convexes on itssurface and functions also as a substrate.
 9. An organic semiconductordevice comprising a substrate, a gate electrode, a gate insulatinglayer, an organic semiconductor layer, a drain electrode, and a sourceelectrode, said organic semiconductor layer comprising a liquidcrystalline organic semiconductor material having a core comprising L 6π electron rings, M 8 π electron rings, N 10 π electron rings, O 12 πelectron rings, P 14 π electron rings, Q 16 π electron rings, R 18 πelectron rings, S 20 π electron rings, T 22 π electron rings, U 24 πelectron rings, and V 26 π electron rings, wherein L, M, N, O, P, Q, R,S, T, U, and V are each an integer of 0 (zero) to 6 andL+M+N+O+P+Q+R+S+T+U+V=1 to
 6. 10. The organic semiconductor deviceaccording to claim 9, wherein organic liquid crystalline moleculesconstituting said liquid crystalline organic semiconductor material arealigned in a direction orthogonal to the film thickness direction of adrain electrode and a source electrode provided on the gate insulatinglayer, and so as to be transversely arranged between the drain electrodeand the source electrode.
 11. The organic semiconductor device accordingto claim 9, wherein organic liquid crystalline molecules constitutingsaid liquid crystalline organic semiconductor material are aligned inparallel with the film thickness direction of a drain electrode and asource electrode provided on the gate insulating layer.
 12. The organicsemiconductor device according to any one of claims 9 to 11, whereinsaid liquid crystalline organic semiconductor material has smecticliquid crystallinity at a predetermined temperature below the heatdecomposition temperature of said liquid crystalline organicsemiconductor material and has a charge mobility of not less than 10⁻⁵cm²/V·s or a hole transport mobility of not less than 10⁻⁵ cm²/V·s.